Electric power supply system

ABSTRACT

Disclosed is an electric power supply system which has: a common power source which stores and supplies a power via a local power grid; and a distributed power source which supplies a power to buildings and a surplus to the common power source. The electric power supply system further includes a first power meter measuring the amount of power supplied from a commercial power source, a second power meter measuring the amount of power received from and supplied to the common power source via the local power grid, and a distribution control device which distributes a power from the common power source to the area based on the amounts of power as measured by the first power meter and the second power meter.

FIELD OF THE INVENTION

The present invention relates to an electric power supply system whichdistributes a power to buildings in a specific area.

BACKGROUND OF THE INVENTION

Conventionally, there has been proposed a technology in which houses aregrouped and power supplied to the houses is cooperatively adjustedtherebetween within the group. Patent Document 1 discloses a technologyfor distributing power supplied from an electric company, wherein apower supply control device is provided between the entire housesincluded in a group and a power system through which the electriccompany supplies power. Further, the power supply control devicemonitors an amount of electric power demand in the houses to adjust thepower supply-demand balance.

In the technology disclosed in Patent Document 1, it is assumed that adistributed power source such as a micro gas turbine power generationdevice, a photovoltaic power generation device, a wind power generationdevice and a fuel cell is provided in at least some of the houses in thegroup. Thus, the power supply control device has a function ofcollectively selling surplus power generated by the distributed powersource to the electric company, and a function of collectivelypurchasing insufficient power of the houses from the electric company.Further, if the power to be supplied to the entire houses of the groupis insufficient, the power supply control device collectively receivespower from the electric company and distributes the power to each of thepower demand houses.

That is, since the power supply control device has a function ofdistributing the power of the distributed power source provided in agroup to the houses of the group, it is possible to reduce the amount ofpower supplied from the electric company.

On the other hand, in Patent Document 2, there has been proposed aphotovoltaic power generating and supplying system for grid connectionincluding a plurality of power generation load units each having a solarcell, a DC to AC conversion unit and an AC load, and a single controlcenter including a common storage cell and a storage control unit, thepower generation load units connecting the single control center.Further, the control center is connected to a power system provided byan electric company. The DC to AC conversion unit has a function ofconverting a DC power generated by the solar cell into an AC power andsupplying the AC power to the AC load and the storage control unit.

In the technology disclosed in Patent Document 2, the control centermonitors the excess or deficiency of power in each of the powergeneration load units, stores power in the common storage cell whenthere is surplus power, and feeds power to the power generation loadunits from the common storage cell when the power is insufficient.Further, the control center detects the amount of the power stored inthe common storage cell, receives power from the power system when thepower stored is insufficient, and causes a reverse power flow to thepower system from the common storage cell when there is a surplus in thepower stored.

In the technology disclosed in Patent Document 2, the solar cellcorresponds to the distributed power source, and the power generationload units can be regarded as the houses on the power demand side.Further, the houses connected to the control center can be regarded as agroup. In other words, in the configuration described in Patent Document2, the distributed power source is provided in the group including thepower demand houses, and the power supply from the power system providedby the electric company and the reverse power flow to the power systemare collectively controlled by the center device, in the same way as theconfiguration described in Patent Document 1.

[Patent Document 1] Japanese Patent Application Publication No.2002-10499

[Patent Document 1] Japanese Patent Application Publication No.2002-233077

As mentioned above, by using the technologies described in PatentDocuments 1 and 2, the power generated by the distributed power sourcecan be used in the power demand houses of the group, and there is aconnection point for grid connection of the power system provided by theelectric company and the power generated by the distributed powersource.

Further, in the configurations described in Patent Documents 1 and 2,since the electric company cannot notice the amount of the powergenerated by the distributed power source or the amount of the powerconsumed in each house, it cannot determine an electricity charge ofeach house. In other words, the amount of the power supplied from thepower system and the amount of the power reversely flowing to the powersystem are perceived only by the power supply control device or thecontrol center relaying between the power system provided by theelectric company and the power demand houses.

In the technologies described in Patent Documents 1 and 2, however, itis possible to perceive the amount of the power supplied to the powersupply control device or the control center and the amount of the powerreversely flowing from the power supply control device or the controlcenter. Accordingly, the electric company can collect electricitycharges based thereon without providing a power meter in each house.

On the other hand, in the power demand houses, the more the amount ofthe power used, the larger the amount of the power accommodated by otherhouse. Therefore, if each house individually makes a contract with theelectric company, it is impossible to employ the technologies of PatentDocuments 1 and 2.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides an electric powersupply system capable of measuring an amount of a power received by eachhouse on the power demand side from a commercial power source whilereducing an amount of a power received from the commercial power source.

In accordance with an aspect of the present invention, there is providedan electric power supply system including: a common power source whichincludes a storage cell, and supplies a power to the storage cell frombuildings and from the storage cell to the buildings via a local powergrid installed in a specific area including the buildings; a distributedpower source which is provided in at least some of the buildings in thearea, for supplying a power to the buildings and supplying a surpluspower to the common power source; a first power meter which is providedin each of the buildings, for measuring an amount of a power suppliedfrom a commercial power source; a second power meter which is providedin each of the buildings, for measuring amounts of power supplied fromand to the common power source via the local power grid; and adistribution control device which distributes a power from the commonpower source to the area on the basis of the amounts of power measuredby the first power meter and the second power meter.

Herein, the local power grid serves as a grid connection of a powersystem of the commercial power source and the respective buildings; andthe common power source has a function of storing a power in the storagecell which is received via the local power grid from the buildings andsupplying a power to the buildings from the storage cell via the localpower grid. Further, the distribution control device performs amanagement so that the amount of the power supplied to the common powersource is coincide with the amount of the power supplied from the commonpower source in each of the buildings, the management includingsupplying a power from the common power source when the amount of thepower measured in a unit period by the first power meter is equal to orgreater than a prescribed value and returning a power to the commonpower source when the amount of the power measured in a unit period bythe first power meter is less than a return value, in each of thebuildings, the return value being a value lower than the prescribedvalue.

With the above configuration, since the common power source shared bythe buildings in a specific area is provided an a power is supplied fromthe common power source to the building in which an amount of a powerpurchased from the commercial power source has increased, it is possibleto suppress the amount of the power supplied from the commercial powersource. For example, if the contract power is set in each of thebuildings, by equalizing the power demand by the common power source, itis possible to prevent the power demand in each of the buildings fromexceeding the contract power.

In addition, since some of the buildings in the area include thedistributed power source, the power generated by the distributed powersource is used in the buildings and a surplus power is stored in thestorage cell provided in the common power source. Accordingly, the powerstored in the common power source can be commonly used in the buildingsin the area. Moreover, an amount of power corresponding to the amount ofthe power supplied from the common power source is returned using thedistributed power source or the commercial power source, so that theamount of the power supplied to the common power source becomes coincidewith the amount of the power received from the common power source ineach of the buildings. Therefore, the amount of the power purchased fromthe commercial power source in each of the buildings is approximatelyequal to that when the common power source is not provided.

Further, a unit price of the power supplied from the commercial powersource may be set in multiple stages, and the distribution controldevice may control charging and discharging of the storage cell based onthe unit price.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become apparentfrom the following description of embodiments, given in conjunction withthe accompanying drawings, in which:

FIG. 1 is a block diagram showing an electric power supply system inaccordance with an embodiment of the present invention; and

FIG. 2 is a block diagram illustrating a case where the building is amultiple dwelling house in the embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings which form a parthereof. Throughout the specification and drawings, like referencenumerals will be given to like parts having substantially the samefunction and configuration, and a redundant description thereof will beomitted.

In the embodiment to be described below, it is assumed that there aremultiple buildings in an area such as a town. Each building may be adetached house, multiple dwelling house or the like. As shown in FIG. 1,at least some of buildings B1 to B3 in the area (hereinafter alsoreferred to as buildings B if it is not necessary to individuallyidentify buildings B1 to B3) include a distributed power source such asa photovoltaic power generation device PV. In the illustrated example,although some of the buildings B include the photovoltaic powergeneration device PV, the photovoltaic power generation device PV may beprovided in each of the buildings B.

The building B1 including the photovoltaic power generation device PV isprovided with a power conditioner 11 which converts an output of thephotovoltaic power generation device PV into an AC power, and allows tomake a grid connection with a power system Pn of a commercial powersource. Each of the buildings B1 to B3 is provided with a distributionboard 12 to supply a power to each load, and the grid connection of thepower system Pn and the power conditioner 11 is carried out in thedistribution board 12.

In the present embodiment, the photovoltaic power generation device PVhas been mentioned as an example of the distributed power source, but apower generation device using natural energy such as wind power, a microgas turbine power generation device, a cogeneration device such as afuel cell to generate power and heat, as long as it can be used as apower generation device, may be used as the distributed power source inany form.

In a case where the building is a multiple dwelling house, as shown inFIG. 2, it is preferable that a storage cell SC1 is provided in additionto the photovoltaic power generation device PV serving as a distributedpower source, and the photovoltaic power generation device PV and thestorage cell SC1 are shared by dwelling units H of a building B4 that isa multiple dwelling house. The storage cell SC1 is provided to store asurplus power when a power is supplied to the dwelling units H from thephotovoltaic power generation device PV. Further, the power stored inthe storage cell SC1 may be supplied to the dwelling units H. To thatend, a charge/discharge circuit 13 is provided to perform charging anddischarging of the storage cell SC1.

In the building B4 of multiple dwelling house, the photovoltaic powergeneration device PV is provided and, accordingly, the power conditioner11 is also provided. The power conditioner 11 converts not only a DCpower from the photovoltaic power generation device PV but also a DCpower from the storage cell SC1. Further, there is provided adistribution board 14 to distribute a power to the dwelling units H inthe building B4.

In the building B4 including multiple dwelling units, the contract powerfor power demand may be set by a collective agreement of the dwellingunits of the building B4. In this case, the power conditioner 11 feeds apower to the dwelling units H from the photovoltaic power generationdevice PV and the storage cell SC1 such that the power demand in thewhole building B4 does not exceed the contract power. In other words,while the photovoltaic power generation device PV generates a power, thepower conditioner 11 supplies the power of the photovoltaic powergeneration device PV to the dwelling units H, and stores a power in thestorage cell SC1 if there is a surplus power. Further, the powerconditioner 11 monitors the power demand in the whole building B4, andfeeds the power of the storage cell SC1 to the dwelling units H when thepower demand reaches a prescribed value that is set on the basis of thecontract power.

Meanwhile, a common power source 2 that is commonly used in the area isprovided, e.g., in the building B3. The common power source 2 includes astorage cell SC and a charge/discharge circuit 21 to perform chargingand discharging of the storage cell SC. Receiving a power in the commonpower source 2 and feeding a power from the common power source 2 areperformed by a distribution control device 22. In the illustratedexample, although the common power source 2 is provided in the buildingB3 such as a municipal hall or a meeting place of the area is assumed asthe building B3, the common power source 2 is not installed necessarilyin such building, and the common power source 2 may be providedseparately.

A local power grid Pd is connected to each of the buildings B1 to B4 inthe area, separately from the power system Pn of the commercial powersource. The local power grid Pd is installed between the distributioncontrol device 22 and the buildings B1 to B4. Accordingly, thedistribution control device 22 has a function of individually performingthe distribution of power between the common power source 2 and each ofthe buildings B1 to B4. Additionally, in the building B3, a power lineequivalent to the local power grid Pd is provided between thedistribution control device 22 and a power meter M2 within the buildingB3.

In each of the buildings B1 to B4, two types of power meters M1 and M2are provided. One power meter, i.e., the power meter M1, measures anamount of a power that has been received from the power system Pn of thecommercial power source. The other power meter, i.e., the power meterM2, measures an amount of a power that has been supplied from/to thecommon power source 2 through the local power grid Pd.

In each of the buildings B1 to B4, the power meter M2 has a function ofmeasuring an amount of a power supplied to the common power source 2 aswell as an amount of a power received from the common power source 2.That is, the power meter M2 of each of the buildings B1 to B4 has afunction of measuring a power in bi-directionally.

The amounts of the power measured by the power meters M1 and M2 areforwarded to the distribution control device 22. That is, thedistribution control device 22 obtains the amounts of the power bycommunicating with the power meters M1 and M2. As a communicationschannel, in addition to a dedicated wired communications channel, thelocal power grid Pd to which the power line carrier communicationtechnology can be employed may be used, and a wireless communicationschannel may also be used.

In this embodiment, a reverse power flow to the power system Pn of thecommercial power source is not considered. Further, when a power outageoccurs in the commercial power source, the power system Pn is removedand an autonomous operation may be performed within the local power gridPd.

With the above-described configuration, even when there are multiplebuildings B1 including the photovoltaic power generation device PV, astorage cell need not to be provided in each of the buildings B1. Thatis, the storage cell SC can be shared by the buildings B1 each includingthe photovoltaic power generation device PV. Therefore, in a system inwhich a power is stored when there is a surplus in the power generatedby the photovoltaic power generation device PV and the stored power isfed when the amount of the power generation is reduced, there is no needto provide the storage cell in each of the buildings B1, and it ispossible to reduce the locations where the storage cell is provided.Further, by commonly using the storage cell, it is possible to reducethe costs compared to a case where an individual resident purchases astorage cell.

Herein, since the storage cell SC is commonly used in the buildings B1to B4, it is necessary to make the amount of the power used in each ofthe buildings B1 to B4 equivalent to that when the storage cell SC isnot commonly used. Accordingly, the amount of the power stored to thestorage cell SC and the amount of the power used from the storage cellSC are measured by the power meter M2, and the measured amount of poweris notified from the power meter M2 to the distribution control device22.

The distribution control device 22 manages the use of the storage cellSC by using the amount of the power measured by the power meter M2 ofeach of the buildings B1 to B4. Further, the distribution control device22 is provided with a local power meter M3 which measures an amount of apower stored in the storage cell SC from each of the buildings B1 to B4using the common power source 2 and an amount of a power supplied toeach of the buildings B1 to B4 from the storage cell SC. The amount ofthe power measured by the local power meter M3 is used in order toobtain an amount of a power that has been lost in the power conversionand transmission.

In other words, ideally, the total amount of the power measured by thepower meters M2 of the buildings B1 to B4 is equal to the amount of thepower measured by the local power meter M3, but actually, there occurs adifference corresponding to a loss in the amount of the power. Bycalculating the difference, the power loss in the use of the commonpower source 2 can be taken into consideration. This makes it possibleto equally distribute the power of the common power source 2 to thebuildings B1 to B4.

Generally, a unit price on electricity charges of the commercial powersource is determined based on the contract power, which has been set onthe basis of a maximum value of the power demand in one month, the powerdemand being an average value of the amount of power used every 30minutes. For that reason, in each of the buildings B1 to B4, it isdesirable to suppress the maximum value of the power demand measured bythe power meter M1.

Then, the distribution control device 22 calculates the power demand ineach of the buildings B1 to B4 by obtaining the amount of power consumedin each of the buildings B1 to B4 being measured by the power meter M1(the amount of power used can be used as a value proportional to thepower demand in case of obtaining the amount of power used every 30minutes). If the power demand in one of the buildings B1 to B4 reaches aprescribed value (an appropriate power value set to be lower than thecontract power), the distribution control device 22 supplies thecorresponding one of the buildings B1 to B4 with a power from the commonpower source 2 to prevent the power demand from exceeding the contractpower.

The power from the common power source 2 can be also supplied to thebuildings B2 and B3 not including the photovoltaic power generationdevice PV as well as the buildings B1 and B4 including the photovoltaicpower generation device PV.

If the building B1 or B4 including the photovoltaic power generationdevice PV has stored a power in the storage cell SC and the stored powerremains, it can be used freely. Further, even if there is no amount ofpower remaining in the storage cell, it is possible to receive a powerfrom the common power source 2 if there is a surplus power in thestorage cell SC. On the other hand, even in the buildings B2 and B3 notincluding the photovoltaic power generation device PV, it is possible toreceive a power from the common power source 2 if there is a surpluspower in the storage cell SC. Thus, in any one of the buildings B1 toB4, it is possible to receive a power from the common power source 2when the power demand reaches the prescribed value, thereby preventingthe power demand from exceeding the contract power.

If a power is stored in the storage cell SC by the buildings B1 and B4including the photovoltaic power generation device PV and the buildingsB1 and B4 uses a larger amount of the power than an amount of the storedpower, it is possible to perform charging of the storage cell SC whenthere is a surplus in the power generated by the photovoltaic powergeneration device PV provided in the buildings B1 and B4. That is, theamount of the power received from the common power source 2 can bereturned to the common power source 2 when there is a surplus power.

Herein, it is desirable to set a deadline such that the amount of thereceived power and the amount of the returned power are offset within anappropriate period. The deadline can be set in units such as one day,one week, one month and one year. If it is impossible to offset thereceived power within the deadline, it is preferable that the powerpurchased from the power system Pn is used for charging of the storagecell SC to offset the received power.

On the other hand, if the common power source 2 has been used by thebuildings B2 and B3 not including the photovoltaic power generationdevice PV, charging of the storage cell SC is performed by the powerpurchased from the power system Pn when the power demand is lowered to areturn value set to be lower than the prescribed value. In other words,in the buildings B2 and B3, it is possible to prevent the power demandfrom exceeding the contract power by using the common power source 2,and charging of the storage cell SC is performed by the power purchasedfrom the commercial power source when the power demand is lowered. Inthis case, the amount of power measured by the power meter M1 is equalto that when the common power source 2 is not used. Therefore, in thebuildings B2 and B3, the increase in electricity charges is suppressedby preventing the power demand from exceeding the contract power, andthe electricity charges commensurate with the amount of power used arepaid to an electric company.

The unit price of electricity charges on the power supplied from thecommercial power source may be set in multiple stages according to timezones of using the electricity. In this case, when a power is used fromthe common power source 2 and then returned thereto by using the powerpurchased from the commercial power source, it is possible to use thepower purchased in the time zone when the unit price of electricitycharges is low. As a result, it is possible to prevent an increase inelectricity charges in each of the buildings B1 to B4 by using the powerpurchased in the time zone when the unit price of electricity charges islow. Also, in general, the power demand is low in the time zone when theunit price of electricity charges is low, and it is possible to equalizethe power supply by increasing the amount of the power used during thistime zone.

As described above, according to the embodiment of the presentinvention, since the common power source 2 is shared by the multiplebuildings B1 to B4 in the area, each of the buildings B1 to B4 can usethe power stored in the common power source 2 if necessary. Inparticular, since the buildings B2 and B3 not including the photovoltaicpower generation device PV can use the power stored in the storage cellSC, it is possible to prevent the power demand from exceeding thecontract power. Accordingly, it is possible to suppress the totalelectricity charges in the area using the common power source 2.

Besides, since the amount of power stored to the common power source 2and the amount of power used from the common power source 2 are measuredby the power meter M2 in each of the buildings B1 to B4, andcollectively managed by the distribution control device 22, the commonpower source 2 can be used as if the storage cell SC is installed ineach of the buildings B1 to B4.

Moreover, in the buildings B2 and B3 not including the photovoltaicpower generation device PV, the amount of the power corresponding to theamount of the power supplied from the common power source 2 is purchasedfrom the commercial power source and returned to the common power source2. Accordingly, the total amount of the power purchased from thecommercial power source and measured by the power meters M1 isapproximately equal to that when the common power source 2 is not used.

While the invention has been shown and described with respect to theembodiment, it will be understood by those skilled in the art thatvarious changes and modification may be made without departing from thescope of the invention as defined in the following claims.

What is claimed is:
 1. An electric power supply system comprising: acommon power source which includes a storage cell and is configured tosupply a power to the storage cell from buildings and from the storagecell to the buildings via a local power grid installed in a specificarea including the buildings; a distributed power source provided ineach of at least some of the buildings in the area and configured tosupply a power to said each of at least some of the buildings and supplya surplus power to the common power source; a first power meter providedin each of the buildings and configured to measure an amount of a powersupplied from a commercial power source; a second power meter providedin each of the buildings and configured to measure amounts of powersupplied from and to the common power source via the local power grid;and a distribution control device configured to distribute a power fromthe common power source to the area on the basis of the amounts of powermeasured by the first power meter and the second power meter, whereinthe local power grid serves as a grid connection of a power system ofthe commercial power source and the respective buildings; and the commonpower source has a function of storing a power in the storage cell whichis received via the local power grid from the buildings and supplying apower to the buildings from the storage cell via the local power grid,and wherein the distribution control device performs a management sothat, during a predetermined period, the amount of the power supplied tothe common power source coincides with the amount of the power suppliedfrom the common power source in each of the buildings, the managementincluding supplying a power from the common power source to a buildingamong the buildings when the amount of the power measured in a unitperiod by the first power meter of the building is equal to or greaterthan a prescribed value and returning a power to the common power sourcefrom the building when the amount of the power measured in the unitperiod by the first power meter of the building is less than a returnvalue, the return value being a value lower than the prescribed value.2. The electric power supply system of claim 1, wherein when a unitprice of the power supplied from the commercial power source is set inmultiple stages, the distribution control device controls charging anddischarging of the storage cell based on the unit price.
 3. The electricpower supply system of claim 1, wherein the distribution control deviceincludes a local power meter to measure amounts of a power supplied tothe storage cell from each of the buildings in the area and a power fromthe storage cell to each of the buildings in the area, and calculates adifference between the amount of the power measured by the second powermeter provided in each of the buildings and the amount of the powermeasured by the local power meter.
 4. The electric power supply systemof claim 1, wherein the power supplied to the common power source fromeach of the buildings includes at least one of the surplus powergenerated from the distributed power source and the power supplied fromthe commercial power source.
 5. The electric power supply system ofclaim 1, wherein the prescribed value is smaller than a contract powerbetween the building and an electric company supplying the commercialpower source.
 6. A distribution control device for use in an electricpower supply system, the electric power supply system including: acommon power source which includes a storage cell, and is configured tosupply a power to the storage cell from buildings and from the storagecell to the buildings via a local power grid installed in a specificarea including the buildings; a distributed power source provided ineach of at least some of the buildings in the area and configured tosupply a power to said each of at least some of the buildings and supplya surplus power to the common power source; a first power meter providedin each of the buildings and configured to measure an amount of a powersupplied from a commercial power source; and a second power meterprovided in each of the buildings and configured to measure amounts ofpower supplied from and to the common power source via the local powergrid, wherein the local power grid serves as a grid connection of apower system of the commercial power source and the respectivebuildings; and the common power source has a function of storing a powerin the storage cell which is received via the local power grid from thebuildings and supplying a power to the buildings from the storage cellvia the local power grid, wherein the distribution control device isconfigured to distribute a power from the common power source to thearea on the basis of the amounts of power measured by the first powermeter and the second power meter, and wherein the distribution controldevice performs a management so that, during a predetermined period, theamount of the power supplied to the common power source from each of thebuildings and measured by the second power meter coincides with theamount of the power supplied to each of the buildings from the commonpower source and measured by the second power meter, the managementincluding supplying a power from the common power source to a buildingamong the buildings when the amount of the power measured in a unitperiod by the first power meter of the building is equal to or greaterthan a prescribed value and returning a power to the common power sourcefrom the building when the amount of the power measured in the unitperiod by the first power meter of the building is less than a returnvalue, the return value being a value lower than the prescribed value.7. A distribution control method for use with an electric power supplysystem, the electric power supply system including: a common powersource which includes a storage cell, and is configured to supply apower to the storage cell from buildings and from the storage cell tothe buildings via a local power grid installed in a specific areaincluding the buildings; a distributed power source provided in each ofat least some of the buildings in the area and configured to supply apower to said each of at least some of the buildings and supply asurplus power to the common power source; a first power meter providedin each of the buildings and configured to measure an amount of a powersupplied from a commercial power source; and a second power meterprovided in each of the buildings and configured to measure amounts ofpower supplied from and to the common power source via the local powergrid, wherein the local power grid serves as a grid connection of apower system of the commercial power source and the respectivebuildings; and the common power source has a function of storing a powerin the storage cell which is received via the local power grid from thebuildings and supplying a power to the buildings from the storage cellvia the local power grid, wherein the method comprises distributing apower from the common power source to the area on the basis of theamounts of power measured by the first power meter and the second powermeter, and wherein the distributing includes performing a management sothat, during a predetermined period, the amount of the power supplied tothe common power source from each of the buildings and measured by thesecond power meter coincides with the amount of the power supplied toeach of the buildings from the common power source and measured by thesecond power meter, the management including supplying a power from thecommon power source to a building among the buildings when the amount ofthe power measured in a unit period by the first power meter of thebuilding is equal to or greater than a prescribed value and returning apower to the common power source from the building when the amount ofthe power measured in the unit period by the first power meter of thebuilding is less than a return value, the return value being a valuelower than the prescribed value.